Dominicans are kind and peaceful people. Attempts at speaking Spanish are a good sign of respect for the local people. Be polite, show respect, and do your best to speak the language, and you will be treated with kindness.
Avoid talking about Haiti. Although relations have improved, many Dominicans, particularly of the older generations, harbor resentment towards Haitians. Santo Domingo was invaded and occupied by Haiti for a good part of the 19th century, and the Dominican Republic actually fought its first war of independence against Haiti, not Spain, after which the Dominican Republic faced several other invasions from its neighbor.
Trujillo's dictatorship massacred tens of thousands of Haitians in the 1930s, which fueled the resentment between both nations. Nowadays, about a million Haitians (which is a lot considering the small populations of either country) live in the Dominican Republic, most of them illegally. Some Dominicans' opinions towards illegal immigrants from Haiti are similar to some Americans' attitudes towards Mexican illegal immigrants, with the major difference that, unlike the US, the Dominican Republic is a small and poor country by world standards, but still much much richer and more stable than Haiti. Gang wars can erupt along the border, so stay cautious and be sensitive.
Still, the issues remain very complex and Dominicans often find their position to be misunderstood by foreigners. For example, the Dominican Republic was the first country to come to Haiti's aid in the 2010 Haitian earthquake and has made impressive efforts to help its neighbor during this crisis. This shows that despite their historical, linguistic, religious, cultural and ethnic differences, Haitians and Dominicans still consider each other to be brotherly, yet proudly independent, nations.
When staying at the luxury resorts or really any place in the Dominican Republic, it is advisable to tip for most services. The Dominican Republic is still a fairly poor country and tipping the people who serve you helps them better their sometimes dire economic situation.